Power-off electromechanical brakes are known in which electrical current to the brake serves to release the brake, which is otherwise activated. Such power-off brakes may be used when it is desirable that the brake be applied in the event of power failure, for example.
In one design of a power-off brake, a friction disk is coupled to the shaft to be braked while allowing some axial movement of the friction disk. The friction disk is normally compressed between a pressure plate and an axially-movable clapper by springs to provide a braking action. An electromagnet may be energized to retract the clapper plate against the force of the springs and allow the friction disk free rotation.
There may be situations where it is necessary to move the shaft of such a power-off brake when power has been lost. In this case, a manual release lever may be provided. The release lever may have a portion fitting around the periphery of the friction disk between the clapper and pressure plate. Rotation of the release lever may engage ramps on the lever and the pressure plate causing the lever to lift the clapper away from the friction disk.
A power-off brake incorporating such a release lever is manufactured by the Inertia Dynamics Division of Rockwell Automation of Connecticut under the tradename "FSBR (Mechanical Release)."
The release lever moves the clapper by a small amount limited by the air gap between the clapper and the electromagnet which is small to maximize the force between the clapper and the electromagnet. Nevertheless, the release lever ideally should move the clapper completely from any contact with the friction disk. This requires that the release lever be manufactured to close tolerances making it costly to produce. The close tolerances of the release lever also make it sensitive to wear, in particular between the ramps on the release lever and the pressure plate such as may prevent the release lever from fully releasing the clapper fully from the friction disk.